We have purified a number of fragments of 16S rRNA from Tl-RNase-treated ribosomes of E. coli, A vinelandii, and B. stearotherm philus, which appear to be derived from identical regions in their respective 16S rRNAs. We intend to determine the primary structure of these regions, by the new methods of end-labeling RNA fragments, partial chemical cleavage, and gel separation of cleaved fragments. The production of identical-sized RNA fragments by nuclease treatment of 30S ribosomes from different bacteria suggests a common surface organization for 16S rRNA. Sequence studies will help us evaluate whether these common surface features are based on common primary structures or semi-conserved primary structures which lead, nevertheless, to common conformations.